An underground fire sent smoke billowing up through manholes at a SoMa District intersection and knocked out power for about 1,100 people Thursday afternoon.

The fire, which was first reported at 1:48 p.m., occurred at the intersection of Folsom and Harriet streets, according to fire department spokeswoman Lt. Mindy Talmadge. Firefighters doused the blaze with CO2, and by 3 p.m. it had been contained.

What exactly caught fire and what caused the blaze are still under investigation, according to Joe Molica, spokesman for Pacific Gas and Electric, which has equipment at the site of the blaze.

Initially, 1,100 people in the SoMa District and Potrero Hill lost power because of the blaze, but as of 3:15 p.m., all but 283 residents had their energy restored, Molica said. A generator has been transferred to the area to power up those residents who are still without power while PG&G investigates what happened, Molica said.

wreisman@sfexaminer.com